How could LaTeX replace the tokens

How could LaTeX replace the tokens

How could LaTeX replace the tokens <= by the command \leq efficiently?
Example 1: I have this code:
\[
2x <= 4x - 2
\]
And I want to get after the compilation this:

Example 2:
\[
q --> q
\]
Output:

Example 3:
\[
Q ==> Q
\]
Output: